1. Field of the Invention
This invention relates generally to an improved method for removal from polyvinyl chloride resins of trapped vinyl chloride monomer that has not been polymerized.
2. Prior Art
In the production of vinyl chloride homopolymers and copolymers, there is absorbed or otherwise retained in the obtained resin, a certain amount of unpolymerized monomer which may vary with the method or conditions employed in the polymerization. Not only is the residual monomer an undesirable contaminant from the standpoint of impairing the quality of the resin, but also there is current belief that such residual vinyl chloride monomer may present a health hazard to persons utilizing the contaminated resin in industry.
Various methods have been heretofore suggested for removal of the residual vinyl chloride monomer and accompanying comonomers from vinyl chloride polymerizates. These prior methods have not been regarded as wholly satisfactory in practical application.
Among the known previous methods attempted or employed is that of steam stripping, an improvement on which is asserted in German patent publication OLS No. 2,331,895. In accordance with the disclosure in this German publication, polyvinyl chloride resin is heated at a temperature between its glass transition temperature (about 67.degree.-68.degree. C) and 180.degree. C, by direct condensation of water vapor thereon, for sufficient time to remove the major portion of the contained monomer, then the condensed water and accompanying monomer are volatilized from the resin by pressure reduction. In the example applied to a polyvinyl chloride homopolymer obtained by gas phase polymerization and which contained 3.4% residual vinyl chloride monomer, about 91% removal of monomer is stated to have been effected.